1. Field of the Disclosure
This invention is in the field of forming a durable release surface by applying a primer powder to a substrate to form a fluoropolymer primer layer thereon, and applying a fluoropolymer powder on the primer layer to form an overcoat. In particular, the invention is directed to the selection of a fluoropolymer primer powder that achieves good intercoat adhesion with a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer powder overcoat and maintains a long lasting bond with the substrate.
2. Background of the Related Art
Fluoropolymer resins having properties such as good chemical resistance, excellent release, good heat resistance and electrical insulation are desirable in a number of applications. Fluoropolymer powders which are melt-flowable have been found useful in coating cookware articles such as rice cookers, grills and bakeware, as well as numerous industrial applications such as fuser rolls or belts for copiers and printers, and chemical processing reactors. One of the advantages of applying powder coatings in lieu of liquid coatings is that the drying and venting steps used in applying liquid coatings, as well as the equipment associated with applying liquid coatings, are eliminated. In addition, powder coatings do not require the use of volatile organic solvents that present environmental concerns and necessitate expensive remediation procedures.
Powder coating, for both a primer layer and an overcoat, is described in U.S. Pat. No. 5,093,403 to Rau et al. In this patent, perfluoroalkoxy polymer (PFA) is exemplified for both the primer layer and the overcoat. This patent recognizes that it is difficult to bond PFA resin to metal substrates, and that PFA must be applied at relatively high temperatures—i.e., in the range of about 675° to about 720° F. (357° to 382° C.). Rau et al. discloses the use of binders such as poly(phenylene sulfide) (PPS) to achieve bonding of the PFA resin to the metal substrate at these elevated temperatures without any significant deterioration (degradation) to the PFA.
Because of its high service temperature, good abrasion resistance and excellent release properties, PFA is the resin of choice for surfaces used in rigorous commercial applications such as for release surfaces for commercial bakeware. Commercial bake pans undergo numerous high temperature cycles each day and must retain their release properties for a significant length of time to make commercial production of baked goods economical. However, experience has shown that the application of a PFA overcoat on a PFA primer layer results in inadequate adhesion of the system over time. As a result, a PFA/PFA system as disclosed in Rau et al. may fail too quickly and inadequately addresses the needs of a commercial operation that subjects substrates with release surfaces to thousands of bake cycles per year.
Thus, there remains a need for an improved primer powder composition that can be used with a PFA topcoat, and that will permit a primer/topcoat system that can be used at high service temperatures with improved adhesion and longer life while maintaining good release properties and abrasion resistance.